Alcohols are of use in many industries including the perfume, pharmaceutical and fuel industries. Ethanol and butanol, for example, are rapidly becoming major liquid transport fuels around the world. Processes for the production of various alcohols are known. Industrial alcohol production is largely synthetic, deriving from petrochemical processes. However, microbial fermentation can also be used to produce alcohols, for example biofuels, and is becoming increasingly popular.
Biofuels for transportation are attractive replacements for gasoline and are rapidly penetrating fuel markets as low concentration blends. Biofuels, derived from natural plant sources, are more environmentally sustainable than those derived from fossil resources (such as gasoline), their use allowing a reduction in the levels of so-called fossil carbon dioxide (CO2) gas that is released into the atmosphere as a result of fuel combustion. In addition, biofuels can be produced locally in many geographies, and can act to reduce dependence on imported fossil energy resources. Alcohols suitable for use as biofuels include ethanol, butanol and 2,3-butanediol, among others.
Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2005 was an estimated 12.2 billion gallons. The global market for the fuel ethanol industry has also been predicted to continue to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA and several developing nations.
For example, in the USA, ethanol is used to produce E10, a 10% mixture of ethanol in gasoline. In E10 blends, the ethanol component acts as an oxygenating agent, improving the efficiency of combustion and reducing the production of air pollutants. In Brazil, ethanol satisfies approximately 30% of the transport fuel demand, as both an oxygenating agent blended in gasoline, or as a pure fuel in its own right. Also, in Europe, environmental concerns surrounding the consequences of Green House Gas (GHG) emissions have been the stimulus for the European Union to set member nations a mandated target for the consumption of sustainable transport biofuels.
Butanol may also be used as a fuel in an internal combustion engine. It is in several ways more similar to gasoline than it is to ethanol. As the interest in the production and application of environmentally sustainable fuels has strengthened, interest in biological processes to produce butanol (often referred to as bio-butanol) has increased. Butanol may be produced by microbial fermentation of biomass from crops such as sugar beet, corn, wheat and sugarcane. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed and the cultivation of starch or sucrose-producing crops for butanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into fuel butanol.
Anaerobic bacteria, such as those from the genus Clostridium, have been demonstrated to produce ethanol from CO, CO2 and H2 via the acetyl CoA biochemical pathway. For example, various strains of Clostridium ljungdahlii that produce ethanol from gases are described in WO 00/68407, EP 117309, U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438. The bacterium Clostridium autoethanogenum sp is also known to produce ethanol from gases (Abrini et al, Archives of Microbiology 161, pp 345-351 (1994)).
However, ethanol production by micro-organisms by fermentation of gases is typically associated with co-production of acetate and/or acetic acid. As some of the available carbon is converted into acetate/acetic acid rather than ethanol, the efficiency of production of ethanol using such fermentation processes may be less than desirable. Also, unless the acetate/acetic acid by-product can be used for some other purpose, it may pose a waste disposal problem. Acetate/acetic acid is converted to methane by micro-organisms and therefore has the potential to contribute to GHG emissions. Other waste products, such as butyric acid/butyrate may also be produced during commonly used fermentation processes.
Furthermore, biological fermentation processes, which typically utilise yeast or bacteria, may be limited to the production of one or two alcohols which a particular organism is able to produce from the substrate on which it is grown (for example a carbohydrate or gas comprising carbon monoxide). If one wishes to produce a different alcohol, a different micro-organism may be required to be sourced. In many cases, one may not be able to source a bacteria capable of producing the desired alcohol. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources such as organic acids, into desirable products, such as fuel ethanol.
Processes for the microbial conversion of acids to their corresponding alcohols have been described previously: U.S. Pat. No. 4,851,344; White and Simon, Arch Microbiol (1992) 158: 81-84; Huber et al, Arch Microbiol (1995) 164: 110-118. However, these methods also suffer from a number of disadvantages. For example, they require the use of chemical mediators many of which are toxic and/or expensive. In addition, the methods use cell extracts, or isolated cells which are dormant and are required to be spun down and resuspended in buffer prior to conversion of the acids to alcohols. These processing steps are labour intensive and increase the risk of the microbes being exposed to oxygen, lysed or otherwise damaged.
The present invention provides processes for producing valuable alcohols by anaerobic bacterial fermentation that overcome certain disadvantages of the methods known in the art, or at least to provide the public with a useful choice.